fix: read request body correctly

[savonarola]

No description provided.

[keynslug]

Just wanted to chime in with previous experience.

Deep improper lists may show slightly better performance, likely due to less GC pressure. I.e.

Suggested change

	{more, Bin, NewRequest} -> binary_decoder(NewRequest, [Bin | Acc]);
	{ok, Bin, DoneRequest} -> {ok, iolist_to_binary(lists:reverse([Bin | Acc])), DoneRequest}
	{more, Bin, NewRequest} -> binary_decoder(NewRequest, [Acc | Bin]);
	{ok, Bin, DoneRequest} -> {ok, iolist_to_binary(Acc)), DoneRequest}

GC Pressure @ 10k chunks of 400 bytes each

emqx_binacc_bench:accum_deep(Es)         emqx_binacc_bench:accum_reverse(Es, [])
                                         
minor_gcs:                  100          184
minor_gcs_duration:         773          5247
major_gcs:                    0          0
major_gcs_duration:           0          0
heap_reclaimed:         2100919          2907062
offheap_bin_reclaimed: 50002500          50002475
stack_min:                    5          4
stack_max:                    5          6

Admittedly, this is pretty hard to prove in practice: simple synthetic microbenchmarks fluctuate between ~40% improvement in runtime to slight negative effect with around 10-15% improvement on average, depending on the choice of parameters. Which probably hints at memory allocation machinery complexity. So take it with a grain of salt.

eministat @ 10k chunks 400 bytes

x accrev / Fixed List 10k
+ deep / Fixed List 10k
+--------------------------------------------------------------------------+
|++*********xxxxx         xx xxx xxxxx xxxx xxxxx   x                     x|
|+++****** *xx  x            x x x xxx xx x xxxx                           |
|+++**+*** x x                 x x x x  x x  x                             |
| +++*+*** x                   x x   x  x    x                             |
|   +*+**x                     x x   x  x                                  |
|   +*+**x                       x   x                                     |
|   +++**x                                                                 |
|   +++*+                                                                  |
|   +++++                                                                  |
|   +++++                                                                  |
|   +++++                                                                  |
|   + +++                                                                  |
|   + ++                                                                   |
|     ++                                                                   |
|     ++                                                                   |
|     ++                                                                   |
|     ++                                                                   |
|     ++                                                                   |
|     ++                                                                   |
|     ++                                                                   |
|     ++                                                                   |
|     ++                                                                   |
|     +                                                                    |
|     +                                                                    |
|       |_______________A__M____________|                                  |
|   |_A_|                                                                  |
+--------------------------------------------------------------------------+
Dataset: x N=100 CI=95.0000
Statistic     Value     [         Bias] (Bootstrapped LB‥UB)
Min:            1859.00
1st Qu.         2335.00
Median:         4175.00
3rd Qu.         5221.00
Max:            8858.00
Average:        3908.50 [     0.290371] (      3597.91 ‥       4228.45)
Std. Dev:       1602.88 [     -9.82188] (      1489.09 ‥       1882.22)

Outliers: 0/0 = 0 (μ=3908.79, σ=1593.06)
	Outlier variance:      0.989089 (severe, the data set is probably unusable)

------

Dataset: + N=100 CI=95.0000
Statistic     Value     [         Bias] (Bootstrapped LB‥UB)
Min:            1620.00
1st Qu.         1978.00
Median:         2137.00
3rd Qu.         2248.00
Max:            2566.00
Average:        2110.53 [    -0.248106] (      2071.25 ‥       2149.37)
Std. Dev:       196.815 [     -1.41853] (      171.619 ‥       227.719)

Outliers: 0/0 = 0 (μ=2110.28, σ=195.397)
	Outlier variance:      0.769069 (severe, the data set is probably unusable)

Difference at 95.0% confidence
	-1797.97 ± 316.524
	-46.0015% ± 8.09836%
	(Student's t, pooled s = 1141.92)
------

[savonarola]

Thanks for the insights, eministat visualizes significant difference, I changed the implementation.

However, this code is probably almost never called in practice 😅